: : : "Sally ports" were a feature of castles and fortresses,
a closely-guarded opening or door in the wall of a fortified building designed
for the quick passage of troops. One of the primary uses of these doors was to
mount quick attacks on whatever enemy army might be besieging the castle at the
moment, and here's where we meet "sally." A "sally," from the Latin "salire" meaning
"to jump," was originally a sudden rush out of a besieged position, a lightning
attack designed to surprise the enemy. "Sally" in this original sense first appeared
around 1560, and "sally port" is first found around 1649. "Sally" has since acquired
the broader sense of "an excursion or escapade." And since castles and fortresses
are in short supply these days, "sally port" has gradually come to mean any guarded
doorway or opening.

: : ...which perfectly answers the original question. To
sally forth is relatively commonly used, albeit usually when the speaker wants
to be deliberately archaic.

: : Now's what's the origin of "Tally Ho"?

: Ah,
thank goodness for the South where archaic phrasing still occurs. Sally forth
and sally out are still used quite commonly, espically in Louisiana. I have sallied
forth from many a banquette after receiving lagniappe.

: As to Tally Ho . .
.
: Two hundred years ago, according to a magazine of that date, the English
fox-hunter's cry was
: " Tallio, Hoix, Hark, Forward," which is a corruption
of the French hunter's call. Four hundred years ago the French hunter encouraged
his dogs with the musical cry of "Thia-hilaud a qui forheur!" sometimes printed
"Tya-hillaut a qui forheur!" (These huntsmen's shouts are given in a quaint and
rare old French book illustrated with the strange pictures of the day and entitled
"La Venerie de Jacques du Fouilloux, a Paris 1573.") From this the English manufactured
"Tallio, hoix, hark, forward." Later it has been abbreviated to simply "Tally-ho."

Hmmm,
I am deeply cynical. I found that the above pasted explanation of the origins
of "tally ho" (from www.inquiry.net) was the only such reference on the web. Many
many moons ago, I accidentally found myself studying French at University, including
the tortuous mediaeval variety. Admittedly the Chanson De Roland is a tad tricky
to translate, but by 1573, even allowing for the supposed hunting cry to be archaic
by that stage, the language is pretty recognisable. Anyone with a good working
knowledge of modern French would not have too great a trouble reading the plays
of Corneille or Racine, for example, which were written a mere 80 or so years
later. The word formations just look wrong, especially with "thia" or "tya".

As
a complete side note, one of my Professors who had extreme privileged access to
the Bodleian library, once asked to be allowed to study one of the very earliest
copies of the Chanson De Roland. It dates from the 12 or 13th century, is hand-quilled
onto vellum, and is normally stored under a high-security perspex case in the
library, but since this was a published mediaeval history don asking to see it,
they let him. The robust tweed-clad librarian of a certain age in the elite reading
room looked up a few minutes later to see (with absolute horror) the crusty professor
annotating the margins of the ancient tome with red ballpoint pen. She screamed
and threw herself at him, preventing any more notation, whereupon security rushed
in and dragged the errant sage out, kicking and screaming and protesting that
he ought to be allowed to do this. Needless to say, his library privileges were
rapidly and permanently revoked. Mind you, this was the same professor who once
held an entire hour-long tutorial with me whilst lying supine on a shelf in a
bookcase 4 feet up a wall, like an effigy on the tomb of a crusader, so perhaps
the signs were there earlier for those to see.

The older English universities
remain encouragingly bizarre
and on reflection, I hope they always do. It's
character-forming :)